Methods and apparatuses are known for manufacturing sacks made of polyethylene or other heat-weldable material, starting from two sheets, unwound from respective rolls, which are superposed on each other and are conveyed with a longitudinal movement, along a vertical path of travel along which they are held at their longitudinal edges by pairs of superposed chains which are synchronized with each other so as to transport downwards the tube formed by the two sheets. Stationary guides control the active runs of the chains to firmly clamp the edges of the sheets. Usually, said chains are advanced intermittently, and during the dwell period the superposed portions of the two sheets, which are disposed outside of the chains, will be engaged by pairs of rectilinear, parallel and opposite welders which effect longitudinal and continuous heat-welds on said sheets so that a continuous tube exits from the lower portion of the pairs of chains and is then closed by a unit for transverse sealing and intermediate cut.
The known art teaches also how to pleat longitudinally, with symmetrical configuration, the longitudinal edges of at least one of the sheets constituting the tube, whereby the sacks made therefrom will have a bellows-shaped or folded cross section.
The longitudinal welding according to the conventional art is effected in such a manner that the two sheets are caused to adhere to each other due to local melting. When the wall of a sack is subjected to traction, a weld effected as described above often leads to the rupture of the wall even with traction forces much lower than those which would have been supported by a not welded sheet, so that sheets of comparatively high thickness must be used in order to obtain strong welded joints. The greater thickness of the sheets, obviously, involves higher costs, as a result of the longer time required for the welding operations. The poor resistance of the longitudinal sealings according to the conventional art is mainly due to the fact that the force exerted by the sacked product on the welded zone is perpendicular to the zone which, therefore, is subjected exclusively to traction.
In a sack manufactured according to the conventional art, the welded edges protrude considerably from said sack and dispose themselves perpendicularly thereto, thus originating problems of space requirements, safety and outer appearance.